According to a conventional method for processing a stencil master plate for stencil printing, an original image is photo-electrically scanned with an image sensor, and the density of the image is converted into a binary signal for each pixel so that the heat sensitive film of thermo-plastic resin of the thermal stencil master plate may be perforated in the manner of a dot matrix by selectively heating each of the minute heat elements of a thermal head according to the obtained binary digital signal representing the image.
In such a method for processing a stencil master plate, when the image signal is converted into a binary signal according to a fixed threshold level, for instance, in case of a character image, all of the minute heat elements of the thermal head corresponding to the region judged to be "black" are heated, and each and every dot in such a region of the film is perforated with the minute heat elements.
Conventionally, perforation of the heat sensitive film with the minute heat elements of the thermal head is carried out without regard to the size, shape or position of the region which is judged to be "black". Therefore, in the black region or the solid dark region extending both in horizontal and vertical scanning directions, the minute heat elements of the thermal head are continuously driven, and this may lead to an over-heated condition. In this case, an amount of heat that is more than required for the perforation on the heat sensitive film is applied, and accordingly the heat sensitive film is subjected to an excessive heat shrinkage for the intended size of perforations.
In such a case, and in the solid region, the gaps between the perforated dots on the heat sensitive film may totally disappear, i.e., the perforated dots may be merged with each other. Therefore, an excessive ink deposition on the printing paper in this region and the problem of offsetting may occur.
Further, the part of the heat sensitive film situated in the gaps which are almost disappeared by excessive heat shrinking between perforated dots may be locally torn apart in its molten state from the adherence to the support of the thermal stencil master plate, and may clog the perforations up by sticking to the fibers of the support which the molten film encounters during the process of thermal perforation. This may cause localized loss in density or blur in the printed image.
Also, since the heat emitting condition of the minute heat elements of the thermal head may vary from one to another depending on the pattern of the image, the shape and the perforating efficiency may vary from one point on the stencil master plate to another, and the images of solid or fine characters may not be reproduced on the printing paper in a satisfactory fashion.